Sample introduction for detection/quantification of trace elements by atomic spectrometry has relied greatly on the use of solution nebulization techniques. This is wasteful of sample and has an efficiency of typically about 2%. In addition, the entire sample is admitted to the detector volume (whether it is a graphite furnace for AAS or an Ar carrier gas for ICP-OES or ICP-MS) carrying with it the matrix constituents and potential interferences.
Vapour generation for sample introduction has enjoyed widespread use for some 40 years, serving to enhance detection limits, improve sensitivity, alleviate matrix and spectral interferences, and foster throughput when coupled to atomic spectroscopy (Sturgeon 2002; He 2007). By resorting to vapour generation, the efficiency can approach 100% with high selectivity for the analyte and thus reduced interferences, higher sensitivity and better limits of detection. However, conventional hydride and cold vapour generation, the most popular vapour generation techniques, are limited to semi-metals and mercury, respectively. Full advantage of the techniques cannot be realized because of the small scope of elements amenable to vapour generation.
Recently, the scope of elements amenable to chemical vapour generation (CVG) by reaction with sodium tetrahydroborate has expanded from the classical suite characterized by a limited number of elements comprising Groups 14-16 of the Periodic Table to include several transition and noble metals, engendered by the realization that rapid gas-liquid phase separation of the potentially unstable reaction products (Sturgeon 2002; Pohl 2007) not only permits their detection, but can significantly reduce the impact of matrix interferences on conventional hydride generation (HG) (Ding 1997; Tao 1999; Moor 2000). With these principles and goals in mind, a variety of approaches to CVG have followed suit, including use of a movable reduction bed generator (Tian 1998), a dual concentric pneumatic nebulizer for rapid mixing of reductant and sample (Moor 2000), a multi-mode sample introduction spray chamber (MSIS) incorporating ports for both nebulization and CVG capabilities (McLaughlin 2002), a direct hydride generation nebulizer for simultaneous introduction of reductant and sample (Rojas 2003), a dual port modified cyclonic spray chamber for similar purposes (Maldonado 2006), and a triple mode introduction system capable of CVG (Asfaw 2007).
Chemical vapor generation (CVG) is a widely adopted sample introduction method for analytical atomic spectrometry in which precursor compounds (usually ionic, metallic or organometallic species) can be transferred from the condensed phase to the gas phase, yielding the advantages of efficient matrix separation, high analyte transport efficiency, high selectivity, simple instrumentation, and ease of automation. Hydride generation enjoys the greatest popularity as a consequence of its ease of implementation, fast reaction and high yield.
Photochemical vapour generation (PVG), a newly emerging research field in analytical chemistry (He 2007; Guo 2003; Guo 2004b), may provide a powerful alternative to conventional CVG due to its simplicity, versatility and cost effectiveness. Although photocatalytic pre-reduction has been used for a number of years, the most attractive aspect of this newly emerging area is the direct generation of volatile species using photochemical reactions. Flow-through and batch reactors employing low molecular weight (LMW) organic acids as photochemical agents for such systems as mercury and selenium have been studied recently (He 2007).
However, there remains a need in the art for improved apparatuses and processes for vapour generation of an analyte from a sample, especially for photochemical vapour generation (PVG).